Process of zone hardening steel articles



Dec. 2, 1930. J. R. ADAMS PROCESS OF ZONE HARDENING STEEL ARTICLES FiledJune 12. 1929 Patented Dec. 2, 1930 UNITED STATES PATENT OFFICE JAMES R.ADAMS, OF HATBORO, PENNSYLVANIA PROCESS or ZONE HARDENING TEEL ARTICLESApplication filed June 12,

a the periphery of the roll to a given depth above the hardening pointof the particular material of which the rolls are made without raisingthe remainder of the roll above the hardening point of that material. Infact, it is usually desirable to keep the temperature of the interior ofthe roll as low as possible.

In the process as heretofore practiced, heat is applied to the exteriorof the roll. Such 1.; heat may be and has been applied in oil-firedfurnaces, in gas-fired furnaces and in electric furnaces. The firing isdone as rapidly as practicable with the object of raising the outsideZone to above the critical temperature of the steel before the interiorattains such temperature. In all of these processes, all the heat issupplied from the outside, either by conduction from the hot gaseswithin the furnace, or by radiation from the furnace walls or heatingelements of an electric furnace. It is necessary not only to heat theouter face of the steel substantially above the hardening point, but itis also necessary to hold the face at that temperature for the timerequired to so enable sufficient heat to penetrate the roll until alayer of desired depth is heated to such temperature. In the process,several difliculties are experienced. It is difiicult or impossible tobuild up the desired heat quickly throughout this layer withoutoverheating the surface. It is diflicult or impossible more slowly tobuild up the desired heat throughout this layer without causing theinterior to heat up by conduction to above the hardening temperature.Approximate success depends upon a nice control of the initialtemperature of the walls of the furnace and of the amount of heatsupplied to the furnace per unit'of time, while the particular heatconductivity of the material used is a factor involved in the rate. ofheat penetration which makes certain control and uniformity of productextremely difficult. This varying heat conductivity tends in varyingdegree to heat the interior of the roll to a hardening 1929. Serial No.370,360.

temperature by conveying heat away from the surface. In these furnaces,also, the material is heated in an atmosphere which tends to producesurface oxidation, while the high initial wall temperature necessary isvery se Vere upon the refractories. p

The object of this invention is so to heat the roll or other article tobe zone-hardened that the roll may be heated at and near its surfaceabove the hardening temperature while the interior of the roll remainsrelatively cold, to accurately and certainly pre determine the depth ofthe outer zone to be hardened, to increase the speed of the applicationof heat, to avoid overheating of the surface or the withdrawal ofsubstantial heat from the zone being heated by conduction, and'to carryon the process in an atmosphere adapted to prevent oxidation.

To aid in an understanding of the process, it is described in'connectionwith the accompanying drawing, which shows, in longitudinalcross-section, a roll enclosed in a high frequency electric inductioncoil.

The roll a, which is to be zone-hardened, is peripherally surrounded byany convenient type of coil for electric inductive heating, such asahelical coil 1) connected with a source of high frequency electriccurrent supply. Preferably the coil is artificially cooled by being madehollowto receive a cooling medium. A high frequency coil of thischaracter is set forth in the Northrup Patent No. 1,328,336, January 20,1920. It will be understood, however, that the practice of the processis not limited to the useof a high frequency electric inductive heaterof any particular type.

A current of a power and frequency which should vary dependent on thedepth of the outside layer which it is desired to heat to a hardeningtemperature and the rate at which the heating is to be done, ashereinafter explained, is passed through the coil. Heat is generated inan outside layer of predetermined thickness until its temperature israised above the hardening point, while the interior of the roll is notraised above a comparatively low, non-hardening temperature. Theproduction of this desirable effect is explained in part by the factthat when heat is all the heating is done within the roll itself,

:butit is entirely practicable to quite definitely'predetermine thedepth of the layer to be heated to' the hardening temperature byadjusting the frequency of the current. For example, with a givencurrent frequency'of any definite numberof cycles per second, thehardening heat'will penetrate to a given I fdepth within a time varyingwith the amount of power used. If the current frequency be tion,: thecontrol of -the mat-iccontrol, the

increased, then the depth of penetration of thehardening heat will beless. If the current frequency be decreased, then the depth ofpenetration'of the hardeningheat will be greater." By increasing ordecreasingthe amount of power used the speed with which the exteriorlayer can be raised to the desired temperature wil'lbe respectivelyincreased or decreased. a

Itwill, therefore, be understood that a layer of any desired depthcan beheated at such a rate that this layerwill be above the criticaltemperature of the-steel and at a predetermined hardening temperaturelong before the center of the roll will be even warm.

' A typical temperature 'to which an outside layer ofpredeter'niin'eddepth may be heated in aroll made of chromium steel is 1500 F. -or more,or approximately 100 or more above the critical temperature ofthe steel.It is frequently desirable, before the zone-heating operation,to-preheat'the wholeroll to a relatively low'temperatureu r While thechief advantage-of the process is the roduetion of a roll havinganextewi're by the continuous process.

While the process has been described as applied to the superficialheating of rolls, its possible applications cover a wider field; and itis intended to claim the process for all uses to which it is foundapplicable. Among other uses to which the process may be applied withspecial advantage are the hardening of mandrel bars for the cold drawingof copper, brass and steel tubing, pipe, etc., the heat treatment ofmandrel bars for cold and hot forging or drawing of brass, copper,nickel and steel'tubing, pipe, etc., the hardening and heat treatment ofthe periphery of discs, cutters, etc., and the heat treatment of Whilethe thickness of the'zone which'is raised to the hardening heat isprimarily. governed by the current frequency,'should the optimumfrequency for a zone of desired thickness not be available the effectsof con duction may be used to increase the thickness of the zone bydecreasing the power input and increasing the time of holding.

Having now fully described my invention, what I claim and desire toprotect by Letters Patentis:

The process of heating steel articles as a step in the operation ofzone-hardening, whlch comprises generating the major portion of the heatWithin an outer zone of metal by a high frequency electric inducedcurrent adapted to heat said zone to a hardening tern perature,regulating the s eed' of the heating operation by the rate o the powerinput, and regulating the depth of penetration of the hardening heat bythe frequency of the current and the rate of power input. In testimonyof which invention, I have hereunto set my hand I at- Philadelphia,Pennsylvania, on this 7th day of June, 1929.

JAMES R. ADAMS.

riorly ardened-zonefoflimited'and fairly definite '-depth without theproduction'of a hardened interior, there are other ronounced do theoperactors which de-' advantages, namely, the

termine and limit the depth of the hardened zone, the possibility 'of aplication of autofissibi ity of carrying on 'nd of atmosphere so. as

the process in any to prevent scaling,.and-.the cleanliness "of the Yprocess. 7 '0 "It will be being superficially heated, as described;

above. the critical temperature, is superfi .CIQIly CQOIBd suddenly, asby immersion in or with liquid, to produce permanent 2 ar ening-of theheated surface.

nnderstoodthat the roll,

